LIBS is a well-known analytical technique for determining the constitution of a sample material that involves focusing a laser beam onto the surface of the sample with high enough power density (i.e. irradiance) to vaporize and ionize a small part of the sample material to produce a plasma or spark having an elemental composition representative of the material. Optical emissions from the plasma plume are collected with light collection optics, and the spectral distribution (i.e. intensity as a function of wavelength) of the collected optical emissions is analyzed in a spectrometer that produces information in electronic form describing the spectral distribution. Since atomic and molecular constituents of sample materials have characteristic optical emission spectra, the information produced by the spectrometer forms a fingerprint of the sample material, revealing the constituents of that part of the sample onto which the laser beam was focused. Plasmas and sparks are used interchangeably in this specification.
LIBS provides rapid, in situ, compositional analysis without touching the surface and is now employed in a wide range of applications such as, for example, the monitoring of active agents in pharmaceutical pills, the sorting of materials for recycling, the analysis of soil to determine its impurities and fertilizer content, and the determination of the composition of molten metallic alloys. The major challenge for these industries is increasing productivity, reducing costs, and maximizing benefits from existing equipment.
The elimination of sample preparation allowing rapid and direct analysis is generally extolled as an advantage of LIBS, especially for quantitative analysis. Material surfaces, however, generally comprise an oxide coating layer or a coating layer containing nitrate, slag, paint, oil, etc. that is not representative of the bulk material to be analysed. To use LIBS to analyse the bulk material, it is first necessary to remove the coating layer. Prior art methods are based on site-by-site analysis involving mechanically cutting or boring a hole into the material at one site using a mechanical drill or using a laser to ablate or clean the layer and expose the bulk material beneath, and then performing the LIBS analysis (see Laser cleaning in conservation of stone, metal, and painted: state of the art and new insights on the use of the Nd:YAG lasers, S. Siano et al., Appl. Phys. A (2012) 106:419-446). The site-by-site method has a number of problems that make representative sampling of the bulk material difficult and prevents one from realizing real-time analysis using LIBS.
For example, the energy distribution within the laser beam (typically a near Gaussian mode in many laser systems) used to clean or ablate the coating layer produces cone-shaped craters with non-negligible edge contribution to the ablated mass. The plasma produced by the laser also interacts with the wall of the crater and induces some mixing of material, which complicates the analysis by LIBS and impacts analytical precision and accuracy, in particular in the region close to an interface. Another problem is the limited thickness of the ablated mass by the laser pulses in the nanosecond regime which is in the order of a few tens of nanometers on, for example, metals. Although appropriate to use for cleaning/ablating coating layers having a thickness of a few micrometers, such methods cannot be used for coating layers having a thickness of a few hundred of micrometers or more due to the time required which prevents one from doing a fast analysis by LIBS. Lack of or poor sensitivity as compared to other analytical schemes is also a problem with using LIBS in this context. In fact, the large background emission (continuum radiation) of the hot laser-induced plasmas can mask and reduce the signal-to-noise ratios of the atomic emission signal from the analyte species, resulting in a lack of or poor sensitivity.
Several solutions have been proposed to remedy existing problems. In Vadillo and Laserna (J. Anal. At. Spectrometry, vol. 12, 1997, p. 859), it was proposed to improve the depth resolution of LIBS measurements by using a simple two-lens telescope combined with a pinhole mask to generate a collimated output of a XeCl excimer laser, resulting in a flat energy profile. Beam masking was also proposed to attenuate the shot energy and to eliminate the peripheral irregularity of the beam profile (see Kanicky et al., Fresenius J. Anal. Chem., vol. 336, 2000, p. 228). In US patent application publication no. 20030016353A1, an approach was proposed based on alternating a burst of shots for ablation and second burst focused in the center of the first burst for sampling. This approach, however, deals with depth profilometry at one position and cannot be applied for scanning the surface.
The above-mentioned approaches have failed, among others, to eliminate the interaction between the laser and the wall of the crater. Furthermore, these approaches can only be used for one position and for layers in the order of a few micrometers, which is a problem for samples or materials having non-homogenous or heterogeneous compositions due to difficulties in obtaining representative samples.
Thus, there remains a need for an improved method of removing the coating layer of a sample in order to allow for an accurate analysis of the bulk material realized in real-time using LIBS.